ISLA MUJERES, MEXICO - U.S. and Mexican scientists believe they are close to solving one of the shark world's great mysteries.

They want to know why whale sharks, the largest shark species, gather each year by the hundreds in the teal-blue waters off this Yucatan Peninsula barrier island.

"It's like a fishbowl full of whale sharks," said Robert Hueter, director of the shark research center at the Mote Marine Laboratory in Sarasota, Fla. "We are witnessing a spectacle of nature down there that we don't fully understand."

Hueter, Mexican biologist Rafael de la Parra and a group of other marine scientists are trying to make sense of the big shark reunion, an event dubbed the "afuera." The word means "outside" in Spanish, and it was the name de la Parra initially used to describe to the phenomenon.

"They are showing up in an area outside the area the Mexican government set up for their protection and outside the area we had normally studied them," de la Parra said.

As research coordinator for Mexico's Domino Project (Mexican fishermen call whale sharks dominoes because of their polka-dot spots), de la Parra has been studying whale sharks off the Yucatan for eight years. "These will be some of the most studied sharks in the world," he said.

Scientists have collected data on water temperature, salinity, oxygen content and types of fish eggs and plankton found at the afuera site. The researchers have documented the length and sex of as many of the sharks as possible and have taken genetic samples and photographs to identify individual sharks. They have also attached visual and satellite tags to many of them, the latter being used to electronically track the animals after they leave the afuera site and head for open ocean.

They are just beginning to explain the mystery of the huge sharks, often described as gentle giants. If toothy predators such as great white sharks (Carcharodon carcharias) are the pit bulls of the shark world, whale sharks (Rhincodon typus) have the temperament of Labrador retrievers. They are harmless to humans. Ironically, the biggest shark in the sea dines on some of the ocean's tiniest food offerings, plankton, small crustaceans and fish eggs, sieving them out of the water through devices near their gills.

Whale sharks are pelagic (ocean roaming) and spend most of their lives swimming alone, traversing thousands of miles of open water in search of food and mates. They are found in most of the world's temperate oceans and occasionally show up in large numbers at events known as "feeding aggregations." They have spawned thriving eco-tourism businesses in places as far-flung as Australia, Africa, the Philippines and Belize.

A slow-moving frenzy

Hundreds of whale sharks appear off the Yucatan barrier island of Holbox each summer to feast in a large swath of ocean turned pea green by a massive plankton bloom. But the Holbox sharks are spread over dozens and even hundreds of square miles in often murky waters.

The afuera is extraordinarily different. Here the giant sharks are concentrated in clear water in an area that can be as large as a few square miles or as small as a football field. Visibility in the blue water is often 100 feet. During the afuera, the huge sharks swarm at the surface, their massive mouths agape, vacuuming up a feast of tiny fish eggs in a chaotic, slow-moving feeding frenzy.

"It's easily the most amazing biological phenomenon I've ever seen," said biologist Al Dove, senior scientist at the Georgia Aquarium in Atlanta. "Even as a scientist, it takes you back to an almost childlike state of wonder, a state of absolute awe."